Method for increasing the wet strength of a tissue material.
The present invention relates to a method for increasing the wet strength of a tissue material. The invention further relates to a tissue material which has been subjected to the method according to the invention.
The invention pertains to the field of wet-strong tissue materials which at least comprise cellulose fibres and a wet strength agent, which for example can be a PAE-resin (Poly Amide-Epichlorohydrine) or another suitable wet strength agent.
Furthermore, the invention touches upon the field of methods for radical formation in gas phase at atmospheric pressure, such as corona treatment, plasma treatment, electronic beam treatment, or treatment with UV light.
The invention can be implemented for all tissue materials which comprise cellulose fibres and a suitable wet strength agent.
The expressions xe2x80x9ctissuexe2x80x9d and xe2x80x9ctissue materialxe2x80x9d are commonly used for designating both creped and through-air dried tissue paper. Roughly, tissue materials can be divided into non wet-strong and wet-strong tissue grades. Non wet-strong tissue is utilised, for example, in toilet paper and so-called facial tissue, whereas wet-strong tissue is utilised, for example, in kitchen roll paper, paper towels, napkins, and in industrial wiping materials.
Currently, when manufacturing wet-strong tissue grades, predominantly cationic wet strength resins are used in order the obtain the desired wet strength, which usually is expressed as wet tensile strength, or as a ratio between wet tensile strength and dry tensile strength (relative wet strength).
One commonly occurring type of wet strength agent for wet-strong tissue grades are so-called polyamide-epichlorohydrine resins (PAE-resins). PAE-resins and most other current wet strength agents contain some kind of reactive groups, such as unsaturated sites (double or triple bonds), epoxy-, amino-, hydroxy- or carboxylic groups. In certain conditions, these reactive groups can be caused to react or interact with chemical groups of other wet strength agent molecules, or with chemical groups of cellulose fibres included in a tissue material. This reaction is usually called curing, and results in a crosslinking and network formation which give the tissue material a higher wet strength. Normally, the curing process takes 3-5 weeks and, in certain conditions, this long outing time can give detrimental substances in the tissue material, for example originating from the pulp raw material, a possibility to disturb or prevent the curing process, so that the intrinsic, normal wet strength-increasing effect of the wet strength agent never is reached.
Normally, the curing course proceeds faster at a raised temperature, and it is previously known that a heat treatment provides a tissue material with full wet strength in a shorter time than the 3-5 weeks which are required with storage at room temperature. A forced curing at a raised temperature is often utilised for quality control of tissue, implying that tissue specimens for wet strength evaluation are xe2x80x9cquick-curedxe2x80x9d in an oven, normally at 105xc2x0 C. for 10 minutes. The wet strength level which is obtained by means of a forced curing generally corresponds relatively well to the wet strength level obtained by means of normal cuing at room temperature during one month. In some cases, however, the forced curing has proved to result in higher values than those which can be obtained by means of normal curing at room temperature or at a temperature which is lower than room temperature (for example in a cold warehouse). Therefore, it has been suggested previously that a heat treatment should be utilised in order to obtain a forced curing also in commercial tissue paper production.
Another type of materials, which have found use for example as industrial wiping materials, are so-called spunlace materials which are manufactured by means of needling with high-pressure water jets. This type of fibre fabrics are also called hydroentangled nonwoven materials and usually also comprise other, longer fibres than pulp fibres. The cohesive force in hydroentangled nonwoven materials is primarily friction between the fibres included in the material and, in case cellulose fibres are present, also hydrogen bonds. One problem which may occur with certain types of hydroentangled nonwoven materials is that the fibre-fibre friction, and thereby also the wet strength, is reduced strongly when such a material is wetted in an aqueous liquid.
Accordingly, in WO 96/27044 it is suggested that a hydroentangled nonwoven material should be subjected to plasma or corona treatment with the purpose of increasing the wet strength. According to WO 96/27044, the disclosed treatment provides an increased fibre-fibre friction also in a wet state, something which results in a higher wet strength of the hydroentangled nonwoven material after the treatment.
It is previously known that methods for radical formation in gas phase, such as the above-mentioned plasma or corona treatment, UV light, electronic beam technique, and other methods can be utilised in order to initiate chemical reactions. The underlying mechanisms do not require any heat supply, but create reactive sites by means of forming radicals and/or excited conditions which in themselves are very reactive.
Something which can be perceived as a disadvantage with conventional wet strength agents for tissue materials is the relatively long storage time which is required in order to achieve the full wet strength-increasing effect.
Heat treatment with the purpose of shortening the storage time by means of a faster curing process has proven to be costly and difficult to accomplish in practice.
Furthermore, in certain applications where tissue materials are utilised, it can be desirable with a higher wet strength than which is possible to reach with natural curing.
Accordingly, the first object of the present invention is to provide a method which confers an increased wet strength to a tissue material without any heat treatment being necessary, and which enables a higher wet strength level than with normal curing and minimises the need for a special storage time for curing.
This first object is achieved by means of a method in accordance with claim 1, wherein the wet strength agent included in the tissue material has an intrinsic, normal wet strength-increasing effect which can be achieved by means of a curing course. Thereby, the curing course comprises chemical reactions and/or physical interactions between different reactive sites of the wet strength agent, between different reactive sites of the cellulose fibres, and reaction and/or interaction between the reactive sites of the wet strength agent and the reactive sites of the cellulose fibres, wherein said reactions and/or interactions result in a network of polymers which is kept together by bonds. According to the invention, the method comprises formation of radicals and/or excited conditions with high reactivity in gas phase at atmospheric pressure, wherein the radicals and/or the excited conditions increase the number of said reactions and/or interactions and/or the number of reactive sites and thereby also the number of bonds, so that the strength of the network is increased and the tissue material thereby is conferred an increased wet strength.
A second object of the present invention is to provide a tissue material, which after having been subjected to the method exhibits an increased wet strength.
In accordance with claim 12, this second object of the invention is achieved by means of the tissue material comprising cellulose fibres and a wet strength agent, which has an intrinsic, normal wet strength-increasing effect which can be achieved by means of a curing course comprising chemical reactions and/or physical interactions between different reactive sites of the wet strength agent, between different reactive sites of the cellulose fibres, and reaction and/or interaction between the reactive sites of the wet strength agent and the reactive sites of the cellulose fibres. Thereby, the reactions and/or interactions result in a network of polymers which is kept together by bonds. According to the invention, the tissue material exhibits an increased wet strength after having been exposed to radicals and/or excited conditions with high reactivity in gas phase at atmospheric pressure, wherein the radicals and/or the excited conditions have increased the number of said reactions and/or interactions and/or the number of reactive sites and thereby also the number of bonds, so that the strength of the network has been increased.
xe2x80x9cNormal wet strength-increasing effectxe2x80x9d, as used herein should be understood as the wet strength increase which the tissue material has obtained owing to the wet strength agent at the point in time in question by means of normal curing at room temperature or at a lower temperature than room temperature.